With the substantial increases in both digital (and analog) communications and communications networks, the effects (i.e., attenuation, frequency variations, etc.) of the interconnection cabling (i.e., twisted pair, coaxial, etc.) on the communications signals is becoming ever important. Communications channels typically consist of one-way channels, two-way channels, or may comprise communication network(s) such as wide area networks (WANs), metropolitan area networks (MANs), or local area networks (LANs). Such networks may include token ring systems, ethernet communications, etc. The communications channel or link typically comprises one or more cables. Transmission of the communications signal over a cable (i.e., transmission medium) typically causes distortion of the signal. This distortion generally occurs as attenuation and frequency variations in the signal which are caused by cable length, temperature, interconnection loss, etc.
In order to correct for the distortion, "equalizers" have been designed and incorporated into the receiving end of the communications channel to "equalize" the signal. For equalization, in some applications, a compromise fixed equalizer is employed to compensate the channel in cables and magnetic storage channels. The performance of a compromise fixed equalizer degrades at short and long cable lengths since it is generally optimized for a medium cable length. For magnetic read channels, the performance of a compromise fixed equalizer degrades with fluctuations in the read channel.
Other equalization methods use an adaptive algorithm to automatically adjust the equalizer to compensate for the distortion caused by the cable (communications channel), and include both fixed and variable components. These methods generally require, in some fashion, tuning of the fixed components of the equalizer to compensate for process variations. Such algorithms adjust the variable components to compensate for the channel variations and process variations. Another implementation uses off-chip resistors and capacitors for the fixed components and off-chip PIN diodes for the variable components. The problem with such an implementation is that it is expensive and requires a large amount of area. Another solution uses an adaptive algorithm to tune several poles and zeros of the equalizer separately. However, this requires complex circuitry to calculate gradients, and often will not converge to the optimal solution unless training is used. Still another implementation uses a multiple-loop-feedback method to realize a general purpose analog filter function that can be designed to operate as an equalizer which has a gain that increases with frequency. This type of equalizer requires multiple parameters to adjust, and it also requires a separate variable gain amplifier (VGA). In addition, it also requires fixed components that will vary with the manufacturing process, thus requiring a tuning mechanism to compensate for process tolerances.
Accordingly, there exists a need for a cable equalizer that compensates for variations in the communications signal caused by distortion in the cable and further compensates for the relatively large process tolerances of the components on an integrated circuit. Further, there is needed an adaptive cable equalizer comprising an integrated continuous-time filter for adaptively compensating for cable variations and semiconductor process variations. Also, there is needed a simple and relatively inexpensive control system to adapt the integrated equalizer without training. Such an implementation can be implemented in standard CMOS technology and will also eliminate the need for a variable gain amplifier (VGA) circuit. There is also needed an adaptive equalizer which does not require accurate fixed components that would require tuning. In addition, there exists a need for a high-order integrated adaptive cable equalizer that performs higher order filter functions and that utilizes at most two control parameters.